Automatic control for energizing electrical precipitators and method thereof



3,648,955 Patented Aug- 14, 1962 AUTQMATIC CQNTRQL FUR ENERGHZENG ELECTRTCAL PREQHPITATORS AND METH- QD TEEREQF Larry L. Little, Los Angeles, Caliii., assignor, hy mesne assignments, to Joy Manufacturing (Iompany, a corporation of Pennsylvania Filed June 11, 1956, Ser. No. %,454) Claims. (Cl. 55-2) The present invention relates generally to electrical precipitators of the type used for the purpose of collecting or precipitating particles suspended in a gas stream. More particularly, the present invention relates to means for automatically controlling the energization of the electrical precipitators in a manner to operate them with maximum efiiciency.

Basically, an electrical precipitator comprises one or more pairs of opposing electrodes between which a high voltage electric field is maintained. It is customary practice to ground the collecting electrode of each pair while the other or discharge electrode is connected directly to a source of electric power at high voltage. As the applied voltage is raised from zero, a point is reached at which there is corona discharge at the discharge electrode; and this condition is accompanied by a small but definite electric current between the two electrodes, this current commonly being referred to as the corona discharge current.

An increase in the applied voltage causes an increase in the corona current; and there is eventually reached a point at which intermittent sparking between the electrodes occurs. Since the individual sparks each represent a current flow between the electrodes, this sparking adds to the total flow of current between the opposing electrodes. The total discharge current is thus composed of a fraction caused by corona discharge and another fraction caused by sparks. In general, each spark represents a substantial increase in the instantaneous value of the discharge current flowing between the electrodes; but since each spark is of such short duration, a single spark has but a comparatively small effect in increasing the average value of the total discharge current.

If the applied voltage is raised high enough, sparking takes place between the electrodes at an increasing rate and with greater intensity until eventually conditions become unstable, resulting in breakdown or are over. When an arc occurs between the electrodes, the voltage falls rapidly to a small fraction of its previous value and the total discharge current between the electrodes rises substantially but the collection efiiciency becomes very low.

Corona current is, in general a measure of the ability of the precipitator to charge suspended particles to be collected. The corona discharge without sparking represents a relatively steady state, and the portion of the total discharge current caused by corona discharge is comparatively uniform for any given voltage. Because it is desired to operate the precipitator at the highest practical value of corona current, it is the usual practice to maintain the applied voltage as high as possible without creating the conditions which bring about arcing. Consequently, at the normal optimum operating conditions there is a considerable amount of intermitting sparking. The frequency and intensity of the sparking may be a function of several factors other than the applied voltage, for example, electrode spacing, character of the gases passing through the precipitator, the cncentration and electrical characteristics of the suspended material in the gas stream, the amount of material collected upon the electrodes, and various other conditions. Also, it should be appreciated that the quantity of electricity added to the discharge current by each spark is not constant, since some sparks may be long or of greater intensity than others and so pass a larger quantity of electricity.

The problem of maintaining optimum operating conditions would be relatively simple if the voltage at which are over occurs were always constant; but this value of the voltage continually fluctuates in any given installation, some times changing rapidly and some times rather slowly. Since this voltage at which arc over occurs has no constant value, the precipitator can not be set to operate at optimum conditions by a fixed control.

Thus it becomes a general object of my invent-ion to provide a device for automatically controlling the energization of the electrodes to maintain optimum operating conditions within the precipitator and automatically to chandge the applied voltage in response to a change in operating conditions.

It is also an object of the invention to provide an automatic control of this character which is at all times responsive to excessive sparking in the precipitator so that should sparking become excessive, the applied voltage is lowered immediately.

It is a further object of my invention to provide an automatic control device of this character which is simple and inexpensive in construction and, more importantly, reliable in operation so that it gives reliable and trouble free performance.

These objects and advantages of my invention are obtained by providing an automatic control device of novel design for use in combination with an energizing circuit for precipitators which includes variable voltage regulating means of any of various well known types of apparatus used for this purpose. The automatic control device includes a reversible motor means for driving or operating the variable voltage regulating means in either one of two directions to raise or lower the inter-electrode voltage applied to the opposed electrodes. A first circuit means is provided that energizes the motor means in the direction to lower the electrode voltage promptly in response to excessive sparking between the opposed electrodes. The response of the first circuit means is such that the voltage is reduced to a value at which the sparking is no longer excessive. The automatic control also includes a second circuit means for energizing the motor means in the opposite direction to raise the inter-electrode voltage after the lapse of a predetermined interval of time after the motor means has lowered the voltage. The second circuit means operates independently of the rate of sparking and serves to raise the voltage periodically to or above the optimum voltage.

The first circuit means is connected to the energizing circuit in any suitable manner, as by a step-down current transformer, in order to receive from the energizing circuit a current signal having a basic frequency that is modulated by transient frequencies resulting from the sparks between opposed electrodes. This signal current is processed in a maner to filter out the basic frequency and to pass substantially only current resulting from the transient frequency portion of the original signal. This current is used to charge a capacitor. The capacitor discharges through an output circuit, the rate of discharge being predetermined. This output is used as a control current which, when abve a predetermined minimum, energizes a switch or relay mechanism adapted to energize the voltage regulator dirve in a direction to lower the inter-electrode voltage.

Operation of this first switch means also re-sets a time delay mechanism in the second circuit means. After the time delay has run through its normal period, which may be adjusted to operating conditions, it operates a second switch or relay which energizes the voltage regulator drive in the opposite direction to raise the voltage applied to the electrodes. The voltage is raised in this manner until the sparking between electrodes becomes excessive when, as a result of the increased control current delivered to the first switch means, the voltage regulator drive is again actuated in its original direction. Movement of the voltage regulating means is reversed and the voltage is lowered to a value at which the sparking is no longer excessive.

How the above objects and advantages of my invention, as well as others not specifically mentioned herein, are attained will be better understood by reference to the following description and to the annexed drawing, in which:

FIG. 1 is a diagrammatic representation of an energizing circuit for a precipitator with an automatic control device of my novel design applied thereto, the automatic control being shown as a block diagram;

IG. 2 is a circuit diagram of the signal processing unit of the control device; and

FIG. 3 is a circuit diagram of a simplified form of signal processing unit.

Referring particularly to FIG. 1, the electrical precipitator is shown schematically as comprising collecting electrode 1t) and discharge electrode 11 which oppose each other. The collecting electrode is normally grounded, as at 12, while the discharge electrode is maintained at a relatively high voltage by means of the energizing circuit for the production of corona discharge at the discharge electrode.

The circuit for energizing the precipitator includes rectifying means 14, which is here shown as being a motordriven-rotary rectifier for purposes of disclosure, but other known types of rectifiers may be used. The rectifier is connected to the secondary coil 15a of transformer 15, the primary coil 15]) of the transformer being connected to conductors 16 which are in turn connected to any suitable source of alternating current. This source is ordinarily a commercial source providing power at a commercial volt-age, usually 220 volts or 440 volts, and at commercial frequency, usually 60 cycles per second.

On the primary side of the energizing circuit there is included a suitable means for efiecting variations in the voltage applied between electrodes 16 and 11. In a preferred embodiment of the invention, this voltage regulating means is an induction regulator having fixed and movable coils 18 and 19 respectively. It will be realized that the invention is not limited to an induction regulator but an auto-transformer or any other suitable means for effecting a variation in the applied voltage may be employed. A grid resistor 17 is preferably included in the primary circuit in series with primary coil b.

Also on the primary side of the circuit, but without necessarily being limited to this particular position, there is a sensing device 20, preferably a current transformer. The transformer primary 20a is placed in series with the primary coil of the step-up transformer 15. The primary current, which is a function of the total discharge current, flows through coil 2th: and is reflected in a proportional current in secondary coil 20b.

The energizing circuit as so far described is well known in the art and likewise its components are all well known elements of any suitable design, and they need not be described in detail.

The automatic control system is indicated diagrammatically by the group of blocks in the lower portion of FIG. 1. Voltage applied to electrodes lit and 11 is varied by moving coil 19 of the induction regulator; and this is done by mechanically connecting the coil to some type of suitable motor means, as for an example a reversible motor which is indicated at by the block representing any suitable motor drive for the voltage regulator. Power for the motor means is supplied from any suitable source 26.

Operation of the motor is controlled by two switch or relay means 27 and 28. The elements 27 and 28 are commonly referred to as relays but each includes a switch controlling the supply of operating current to motor means 25. The switch 27 is designated as the lower voltage switch since it operates the voltage regulator drive in a direction to move coil 19 in the direction that reduces the voltage existing between electrodes til and 11. The second switch 28 is designated as the raise voltage switch since it operates the voltage regulator drive in the other direction, that is, it causes coil 19 to move in the direction that raises the voltage maintained between electrodes 10 and 11.

Switch 27 is actuated by a control current received from signal processing unit 30 which receives and processes a signal current received from the precipitator energizing circuit by means of coil 291) connected to the signal processing unit by two conductors 31.

Lower voltage switch 27 is also connected to an electrical timer mechanism 32 in such a manner that the timer is re-set to its starting point each time switch 27 operates the voltage regulator drive. After operation of the regulator drive, the timer runs for a definite time interval, which is preferably adjustable in order to adapt operation of the control system to conditions existing within the precipitator; and at the end of the time interval the timer causes the raise voltage switch 28 to energize the voltage regulator drive in the direction to increase the voltage applied between electrodes ltl and 11.

The switches at 2'7 and 28 may be of any suitable design. Broadly speaking, any type of electro-rnagnetically operated switch means is suitable, although for technical reasons it is preferred to use magnetic amplifiers for these units. Although the term switch may not ordinarily be applied to magnetic amplifiers, it is intended that in this instance the term be construed broadly enough to include magnetic amplifiers or any other type of switching means adapted to control operation of motor means 25. Likewise, timer 32 may be of any suitable design adapted to perform the function of actuating the raise voltage switch after the lapse of a predetermined interval of time following resetting the timer to its zero point.

The signal processing unit is of novel design and is shown in detail in FIG. 2. The signal processing unit receives a current signal from the energizing circuit by virtue of its inductive connection thereto through transformer 20. This sensing device is preferably located in the primary side of the energizing circuit in order to permit working with the comparatively low voltage present on this side. However, the signal processing unit may if desired be coupled to the energizing circuit at some point in the secondary side using a different type of sensing element.

Conductors 31 from coil 2% are connected to two opposite corners of a rectifying bridge 35 in the signal processing unit. The other two corners of the bridge, which constitute the output terminals, are connected to the two terminals of the main storage capacitor 36. One output terminal of the bridge is connected to one side of capacitor 36 by ocnductor 38. The other output terminal of the bridge is connected to the other side of the storage capacitor through conductor 39, choke coil 40, capacitor 41 and half-wave rectifier 42. Capacitor 36 discharges through an output circuit that includes resistor 43 in series with transistor 44 and resistor 56 which is in parallel with the transistor. Capacitor 45 and conductor 46b in series are connected to one terminal of transistor 44. Conductor 46a is connected to the other terminal of the capacitor to become a part of the output circuit of the capacitor. Resistor 43 is in series with rectifier 4-2.

Connected between conductors 38 and 39 and across the output terminals of rectifying bridge 35, is capacitor 47. Series connected between conductors 38 and 39, and in parallel with capacitor 47, are choke coil 48 and variable resistance 49. Choke coils 40 and 48 have a common connection to conductor 39 and therefore divide the current reaching this poirt.

Between coil 40 and capacitor 41, one side of capacitor 52 is connected, the other side of this capacitor being connected to conductor 38. At the other side of capacitor 41 between it and rectifier 42, there is connected halfwave rectifier 53, the other side of the rectifier being connected to conductor 38. The two rectifiers 42 and 53 have a common connection and are elements of a doubler type rectifier circuit. Between rectifier 42 and resistor 43 there is connected one side of main storage capacitor 36, the other side of the capacitor being connected to conductors 33 and 46a, the latter being a part of the discharge circuit of the capacitor. Capacitors 52 and 36 and rectifier 53 are thus in parallel with each other.

It is preferable to provide within the signal processing unit means for amplifying the output, which is the discharge current from capacitor 36. Means for regulating the degree of amplification is preferably included in the amplifier. This is done by providing transistor 44 connected between conductor 46a and the output terminal of resistor 43. Adjustability of the degree of amplification produced by transistor 44 is obtained by inserting variable resistance 56 in the circuit, this resistance being connected between this same terminal of resistor 43 and conductor 46a. A voltage source for the transistor is provided by rectifying bridge 57 which has its output terminals connected across the terminals of capacitor 45. Input to bridge 57 is obtained from the secondary of transformer 58, the primary of this transformer being connected to any suitable source 59 of alternating current.

Output 46 of the signal processing unit is through conductors 46a and 46b one of which has in series with it choke coil 63 connected to one terminal of capacitor 62, the other terminal of the capacitor being connected to conductor 46a. The primary purpose of the choke coil at this location is to provide a high impedance in the output from processing unit 30 to block feedback from a magnetic amplifier at 27. The magnitude of the output current flowing inconductors 46a and 46b is selected according to the operating characteristics of switch 27. Assume that this element is a magnetic amplifier which can be energized to operate the voltage regulator drive by receipt of a control current in excess of one milliampere (1.0 ma). The signal processing unit 30 is then designed, and the values of its component parts selected accordingly, to produce an output current of 1.0 ma. or more when the input current to the signal processing unit is that produced by a sparking condition which is at or near the maximum sparking that can be tolerated. This maximum sparking condition is normally determined by reference to the particle collection eificiency but it may be any arbitrarily assigned value or sparking condition. Sparking above this maximum is considered to be excessive; and it produces a current signal in conductors 31 that results in the drop in the applied voltage between electrodes and 11, as will now be explained.

As previously mentioned, when the voltage applied to electrodes 10 and 11 is maintained at a value below that at which sparking occurs, there is a relatively constant flow of current between the electrodes as a result of corona discharge from the electrode 11. This condition is reflected in the primary side of transformer by a relatively steady current flowing through primary coil a. This current induces an A.-C. signal current of proportional value, as determined by the ratio between the primary and secondary coils of transformer 20, which current becomes the signal current delivered to the signal processing unit. This signal current has a basic frequency, assumed here to be 60 cycles per second, cor responding to the frequency of the current furnished over conductors l6.

Sparks between electrodes 10 and 11 represent surges of current between the electrodes for short intervals of time. The added quantity of current flowing between the electrodes as a result of a spark depends not only on the intensity of the spark but upon its duration. These sparks are reflected individually in the primary side of the energizing circuit by surges of current in the primary of transformer 26. In turn, the surges produce similar but proportionally smaller increases in current in conductor 31. A spark in the precipitator is similar to the discharge of a large condenser and produces in the energizing circuit a temporary increase in precipitator current over and above the steady state corona current. These temporary increases represent an amplitude modulation of the basic frequency current, and this appears in the current carried by conductors 31. These modulations are of lower frequencies than the higher harmonic frequencies of the 60 cycle current. Therefore, the current signal entering the signal processing unit may be considered to be composed of a basic frequency modulated by transient frequencies which are imposed upon this basic frequency by the sparks which occur between electrodes It) and 11. The current at the modulation frequency is passed to capacitor 36 to charge it.

The signal current reaching the signal processing unit first passes through rectifying bridge 35 which eifects a full wave rectification. The output from the bridge is a unidirectional current of fluctuating value. The basic 60 cycle current signal, when rectified by bridge 35, may be considered as being converted to a direct current component plus a ripple current component having a frequency of cycles per second or twice the basic frequency of the input signal plus lesser quantities of higher harmonics. In addition, the output from rectifier 35 also has another component of much lower frequencies representing modulation of the basic frequency by transient frequencies. The direct current component is present at all times, representing a steady state condition. This D.-C. component is diverted through choke coil 48 and variable resistor 49. The 120 cycle ripple is largely, if not entirely, diverted from the circuit through condenser 47 which bleeds off this component. Capacitor 52 in parallel with capacitor 47 performs a similar function and diverts any of the 120 cycle current that passes coil 4&9.

Thus the elements 47, 48, 49 and 52 in the circuit operate as a filter circuit to divert or bleed off the major fraction of the output from bridge 35 which is caused by the signal current reaching the signal processing unit as a result of steady state or non-sparking condition in the precipitator. Capacitor 41 blocks the steady state D.-C. resulting from the rectified steady state A.-C., but passes current as a result of changes in steady state level, i.e., the modulations produced by sparking.

Capacitor 52 in combination with coil 40 and capacitor 47 remove harmonic frequencies of the basic current from being rectified by doubler circuit rectifiers 42 and 53 and prevent said harmonics from reaching the main storage capacitor 36.

The elements of the filter circuit create a 11-0 voltage across conductors 39 and 38 at their connections to coil 43 and resistor 49 respectively, and this voltage is a function of the current in the incoming signal. This D.-C. potential is blocked by capacitor 4].; however changes in DC. potential caused by sparking produce a current through coil 40, capacitor 41 and rectifiers 42 and 53. This current charges main storage capacitor 36.

The quantity of electricity supplied to the main storage condenser is a function of the current reaching the signal processing unit as a result of current surges created by sparks in the precipitator. It will be obvious that the quantity of this current is a function not only of the number of sparks per unit time, that is, the rate of sparking but also the intensity of sparking since a spark of longer duration or greater current flow has more effect in charging condenser 36 than a spark of short duration or lesser current flow.

At the same time that the main storage condenser is being charged, it is being continuously discharged through resistor 43 which is given a high resistance in order to discharge the condenser at a low rate. The rate of discharge is fixed by the characteristics of resistor 43 and the other elements of the circuit, but this does not mean that the discharge is always at a constant rate since the rate of discharge of the capacitor is approximately proportional to voltage.

The current derived from the discharge of condenser 36 is amplified in order to raise the current value of the output from the processing unit. This amplification is accomplished by means of transistor 44, the current discharge from condenser 36 through resistor 43 constituting the control current for the transistor. Output current for the transistor is derived from the transformer 58 and rectifying bridge 57 providing a direct current which is modified by the control current. The value of the resistance at 56 can be changed so that more or less of the current from the condenser is shunted through transistor 44 to effect a corresponding modification of the output over conductors 46a and 46b.

Conductors 46a and 46b, shown diagrammatically at 4-6, are connected to the lower voltage switch 27 which, as mentioned above, is preferably a magnetic amplifier. it is a characteristic of this unit to provide a certain amount of feedback voltages into processing unit 30; and in order to prevent such feedback from interfering with the operation of the signal processing circuit, a high value choke coil as and a capacitor at 62 are provided to block or filter out the feedback current.

As an optional feature, half-wave rectifier 60 may be included. it is connected to the output side of coil 4t and to conductor 46 or to the output side of condenser d5, and by-passes condenser 36. In this position it operates to act as a current limiting means. The resistance values of elements 48 and 49 in the circuit are so selected that when the current passing through transformer coil 2 3a reaches the maximum value at which it is desired to operate the energizing circuit, a suflicient current passes through rectifier 6t to operate the lower voltage relay 27. This circuit prevents the automatic control from raising the precipitator current above the rating of the energizing equipment.

The control exterted by this automatic control system on the energizing circuit will now be explained. Assume that the precipitator is energized at the minimum voltage, that is, with the voltage regulator drive having moved coil is to the position at which transformer 15 produces a minimum voltage. After timer 32 has run for the period of time for which it is set, it energizes raise voltage switch 28 and causes the voltage regulator drive to change the position of coil l) in a direction to increase the voltage applied to the precipitator. This rise in voltage is made slowly so as not to overshoot the optimum operating conditions of the precipitator. It has been found by experience that if the voltage is raised too rapidly it initially reaches a value considerably above the final value for optimum operation and must be reduced. Consequently the apparatus is set to run through the full range of voltage change in approximately 150 180 seconds, though thi specific time is in no way limitative upon the invention.

If a condition of excessive sparking is reached in the precipitator before the voltage produced by transformer 15 has reached its maximum value, the lower voltage switch 27 is energized in response to the excessive sparking to operate the voltage regulator drive in the opposite direction to lower the voltage. This is accomplished in response to the signal current received by the signal processing unit over conductors 31. When the current in primary coil 23a is produced entirely by corona discharge and not by sparks at electrode 11, conditions 1n the precipitator are stable. This steady state condition is reflected by delivery to the signal processing unit of a sea-seas signal current composed entirely of current having a basic frequency of 60 cycles per second and higher harmonies. This current is filtered out by the signal processing unit with the result that there is a minimum output current to the lower voltage switch 27.

Since a certaint amount of sparking between electrodes 19 and 11 occurs as a result of the increase in voltage, the signal current has a component consisting of current at low frequencies which modulate the current at basic frequency, thus increasing greatly the total quantity of current reaching the signal processing unit. The magnetic amplifier serving as switch 27 is designed to be actuated only by a control current in excess of a predetermined threshold value, as for example in excess of one milliampere. Consequently, a control current output from the signal processing unit of less than one milli- "ere effects no change in the operation of the voltage regulator drive.

The values of the circuit components are such that 20 when the sparking current in the precipitator reaches a value which is more than desirable to obtain best collection, the control current output from the signal processing unit rises above the threshold current required to actuate lower voltage switch 27; and as a consequence the voltage regulator drive is energized in a direction to reduce the applied voltage at the precipitator electrodes.

Each time that the lower voltage relay is energized, the output current to the regulator drive also re-sets the timer 3 2 to the zero point. Consequently, after a pre- 30 determined interval of time following energization of the voltage regulator drive in a direction to lower the voltage, a cycle of operation is re-started by the timer operating the raise voltage switch which energizes the voltage regulator drive in a direction to raise the voltage. This in- 3' crease in voltage is continued until sparking becomes excessive, at which time the above cycle is repeated. It will be noted that the lower voltage switch is always able to over ride the raise voltage switch so that if, at any time current flow to the precipitator is in excess of predetermined values, the control device immediately reacts in a manner to cause the applied voltage to be decreased.

There is illustrated in FIG. 3 a simplified form of circuit for the signal processing unit. It diifers from that illustrated in FIG. 2 primarily in the omission of any means for amplifying the discharge current from the main storage condenser and thus increasing the current value of the output from the unit at 46. More particularly, transistor 44, resistor 56, capacitor 4 5, and the current supply elements 57, 58, and 59, have been omitted. The

current limiting rectifier of} may be omitted if desired but is retained in this circuit.

The complete simplified circuit of FIG. 3 is the same at the right hand side of line es as the circuit shown in FIG. 2 and this part of the processing unit is therefore not repeated in FIG. 3. The only changes in the circuit are those appearing at the left hand side of line 65. Variable resistor as in series with capacitor 41 has been added to effect adjustment of the current charging capacitor 36.

The circuit has been further simplified with the omission of capacitor 62. Choke coil 63 and capacitor 36 are relied upon to provide sufficient filtering in the circuit to prevent an objectionable amount of feedback into the signal processing unit from lower voltage switch 27.

65 Also resistor 43 has been omitted, coil 63 and the resistance of switch 27 providing resistance in the output circuit sufiicient to reduce the rate of discharge of condenser 36 to the desired low rates.

The operation of the simplified circuit of FIG. 3 is the same as that already described, except that there is no amplification of the output from the signal processing unit. The main storage condenser 36 is charged in the manner previously described by current passing through the filter elements of the circuit and representing current caused to enter the signal processing unit as a result of sparking between electrodes and 11. Capacitor 36 is continuously discharged through coil 63 and the load 27 at a predetermined rate because of the impedance provided by elements 63 and 2 When this discharge current from the signal processing unit exceeds the threshold value with which lower voltage switch 27 is operated, the switch energizes the voltage regulator drive, as has been described.

From the foregoing description it will be understood that various modifications may be made in the control system and especially in the signal processing unit without departing from the spirit and scope of my invention; and that the same methods of regulating the energizing circuit may be employed without using exactly the same arrangement of elements illustrated herein. Accordingly, it is to be understood that the foregoing description is considered to be illustrative of, rather than limitative upon, the appended claims.

I claim:

1. An automatic control device for use in combination with an energizing circuit for maintaining a high voltage field between opposed electrodes of a precipitator and including variable voltage regulating means for varying said inter-electrode voltage, compnising: reversible motor means driving the variable voltage regulating means to raise or lower the voltage between opposed electrodes; circuit means energizing the motor means in a direction to lower the inter-electrode voltage in response to excessive sparking between opposed electrodes of the precipitator; timer means connected to the circuit means to initiate a cycle of operation after the lapse of a predetermined time interval following energization of the circuit means to lower the inter-electrode voltage; and circuit means actuated by the timer means at the beginning of said operating cycle to energize the motor means in a direction to raise the inter-electrode voltage.

2. An automatic control device for use in combination with an energizing circuit for maintaining a high voltage field between opposed electrodes of a precipitator and including variable voltage regulating means for varying said inter-electrode voltage, comprising: reversible motor means driving the variable voltage regulating means to raise or lower the voltage between opposed electrodes; an integrating circuit responsive to current surges in the energizing circuit to develop an output signal proportional in strength to both the frequency and the intensity of the current surges; switch means energizing the motor means in a direction to lower the inter-electrode voltage in response to said output signal when above a predetermined threshold value; and circuit means energizing the motor means in a direction to raise the inter-electrode voltage after the lapse of a predetermined time following energization of the motor means to lower the inter-electrode voltage.

3. An automatic control device for use in combination with an energizing circuit for maintaining a high voltage field between opposed electrodes of a precipitator and including variable voltage regulating means for varying said inter-electrode voltage, comprising: reversible motor means driving the variable voltage regulating means to raise or lower the voltage between opposed electrodes; an integrating circuit responsive to current surges in the energizing circuit to develop an output signal proportional in strength to both the frequency and the intensity of the current surges; a first switch means energizing the motor means in a direction to lower the inter-electrode voltage in response to said output signal when above a predetermined threshold value; timer means connected to the first switch means to initiate a predetermined time cycle following energization of the switch means to lower the inter-electrode voltage; and a second switch means actuated by the timer means at the end of the time cycle thereof to energize the motor means in a direction to raise the inter-electrode voltage.

4. An automatic control device for use in combination with an energizing circuit for maintaining a high voltage field between opposed electrodes of a precipitator and including variable voltage regulating means for varying said inter-electrode voltage, comprising: motor means driving the variable voltage regulating means to raise or lower the voltage between opposed electrodes; means connected to the energizing circuit to deliver therefrom a signal current having a basic frequency modulated by transient frequencies imposed by sparks between opposed electrodes; means receiving said signal current and filtering out the basic frequency to pass substantially only current having transient frequencies; a capacitor charged by said passed current; means discharging the capacitor at a predetermind rate to provide a control current; and means operated by said control current to energize the motor means.

5. An automatic control device for use in combination with an electrical precipitator having an energizing circuit for maintaining a high voltage field between opposed electrodes of the precipitator and including variable voltage regulating means for varying said inter-electrode voltage, comprising: motor means driving the variable voltage regulating means to raise or lower the voltage between opposed electrodes; means connected to the energizing circuit to deliver therefrom a signal current having a basic frequency modulated by transient frequencies imposed by sparks between opposed electrodes; means reeeiving said signal current and filtering out the basic frequency to pass substantially only current having transient frequencies; a capacitor charged by said passed current; means discharging the capacitor at a predetermined rate to provide a control current; and means operated by said control current when in excess of a predetermined value to energize the motor means in a direction to lower the inter-electrode voltage.

6. An automatic control device for use in combination with an electrical precipitator having an energizing circuit for maintaining a high voltage field between opposing electrodes of the precipitator and including variable voltage regulating means for varying said voltage, comprising: motor means driving the variable voltage regulating means to raise or lower the voltage between opposing electrodes; means connected to the energizing circuit to deliver therefrom a signal current having a basic frequency modulated by transient frequencies imposed by sparks between opposing electrodes; means receiving the signal current and dividing it into components of which one component is substantially proportional to the quantity of current at said transient frequencies created in said signal current by sparks between electrodes; means converting said one component into a control current of varying intensity; and means energizing the motor means in response to the control current when in excess of a predetermined intensity.

7. An automatic control device for use in an electrical precipitator having an energizing circuit for maintaining a high voltage field between opposed electrodes of the precipitator and including variable voltage regulating means for varying said voltage, comprising: motor means driving the variable voltage regulating means to raise or lower the voltage between opposed electrodes; circuit means inductively connected to the energizing circuit to induce in said circuit means a signal current proportional to current in the energizing circuit and having a fluctuating intensity intermittently increased by current flow caused by sparks between opposed electrodes; means receiving the signal current and dividing it into components of which one component is substantially proportional to the increases in intensity of current in said signal current created by sparks between electrodes; means converting said one component into a control current of varying intensity; and means energizing the motor means in response to said control current when in excess of a predetermined intensity.

8. The method of controlling the voltage applied to the electrodes of an electrical precipitator by an energizing circuit that includes the following steps: deriving an electrical signal proportional to the inter-electrode current which si nal has a basic frequency component corresponding to inter-electrode current caused by corona discharge and a modulation component corresponding to inter-electrode current caused by sparking; deriving from said signal an electrical control signal that varies in magnitude with variations of the modulation component only; varying in one manner the magnitude of the voltage impressed by the energizing circuit between the electrodes, and utilizing said control signal to vary the voltage impressed by the energizing circuit between the electrodes in a manner other than said one manner.

9. The method of controlling the voltage applied to the electrodes of an electrical precipitator by an energizing circuit that includes the following steps: deriving an elec trical signal proportional to the inter-electrode current which signal has a basic frequency component corresponding to inter-electrode current caused by corona discharge and a modulation component corresponding to inter-electrode current caused by sparking; deriving from said signal an electrical control signal that varies in magnitude with variations of the modulation component only; varying in one manner the magnitude of the voltage impressed by the energizing circuit between the electrodes independently of said control signal, and utilizing said control signal to vary the voltage impressed by the energizing circuit between the electrodes in a manner other than said one manner of varying when said control signal exceeds a predetermined value.

10. The method of controlling the voltage applied to the electrodes of an electrical precipitator by an energizing circuit that includes the following steps: deriving an electrical signal current proportional to the inter-electrode current which signal current has a basic frequency component corresponding to inter-electrode current caused by corona discharge and a modulation component corresponding to inter-electrode current caused by sparking; deriving from said signal current an electrical control current that varies in magnitude with variations of the modulation component only; varying in one manner the magnitude of the voltage impressed by the energizing circuit between the electrodes independently of said control current, and utilizing said control current to vary the voltage impressed by the energizing circuit between the electrodes in a manner other than said one manner of varying when said control current exceeds a predetermined value.

11. The method or" controlling the voltage applied to the electrodes of an electrical precipitator by an energizing circuit that includes the following steps: deriving an electrical signal proportional to the inter-electrode current which signal has a basic frequency component corresponding to inter-electrode current caused by corona discharge and a modulation component corresponding to inter-electrode current caused by sparking; deriving from said signal an electrical control signal that varies in magnitude with variations of the modulation component only; intermittently varying in one manner the magnitude of the voltage impressed by the energizing circuit between the electrodes independently of said control signal, and utilizing said control signal to vary the voltage impressed by the energizing circuit between the electrodes in a manner other than said one manner of varying when said control signal exceeds a predetermined value.

12. The method of controlling the voltage applied to the electrodes of an electrical precipitator by an energizing circuit that includes the following steps: deriving an electrical signal proportional to the inter-electrode current which signal has a basic frequency component corresponding to inter-electrode current caused by corona discharge and a modulation component corresponding to inter-electrode current caused by sparking; deriving from said signal an electrical control signal that varies in magnitude with variations of the modulation component only; intermittcnty varying in one manner the magnitude of the voltage impressed by the energizing circuit between the electrodes, and utilizing said control signal to vary the voltage impressed by the energizing circuit between the electrodes in a manner other than said one manner.

13. The method of controlling the voltage applied to the electrodes of electrical precipitator by an energizing circuit that includes the following steps: deriving an elec rical signal proportional to the inter-electrode current which inter-electrode current has a basic frequency component corresponding to inter-electrode current caused by corona discharge and a modulation component corresponding to inter-electrode current caused by sparking; deriving from said signal an electrical control signal that varies in magnitude with variations of the modulation component only; increasing the magnitude of the voltage impressed by the energizing circuit between the electrodes independently of said control signal, and utilizing said control signal to decrease the voltage impressed by the energizing circuit between the electrodes when said control signal exceeds a predetermined value.

14. The method of controlling the voltage applied to the electrodes of an electrical precipitator by an energizing circuit that includes the following steps: deriving an electrical signal proportional to the inter-electrode current which inter-electrode current has a basic frequency component corresponding to inter-electrode current caused by corona discharge and a modulation compon nt corresponding to inter-electrode current caused by sparking; deriving from said signal an electrical control signal that varies in magnitude with variations of the modulation component only; increasing by means of a variable induction regulator the magnitude of the voltage impressed by the energizing circuit between the electrodes independently of said control signal, and utilizing said control signal to decrease the voltage impressed by the energizing circuit between the electrodes when said control signal exceeds a predetermined value.

15. The method of controlling the voltage applied to the electrodes of an electrical precipitator by an energizing circuit that includes the following steps: deriving an electrical signal current proportional to the interelectrode current which inter-electrode current has a basic frequency component corresponding to inter-electrode current caused by corona discharge and a modulation component corresponding to inter-electrode current caused by sparking; deriving from said signal current an electrical control current that varies in magnitude with variations of the modulation component only; intermittently increasing the magnitude or" the voltage impressed by the energizing circuit between the electrodes independently of said control current, and utilizing said control current to decrease the voltage impressed by the energizing circuit between the electrodes when said control current exceeds a predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS 1,834,134 Paschen Dec. 1, 1931 2,771,150 Welts Nov, 26, 1956 2,925,142 Wasserman Feb. 16, 1966 FOREIGN PATENTS 644,756 Germany Nay 12, 1937 670,245 Germany Jan. 14, 1939 705,604 Great Britain Mar. 17, 1954 

